With the increase in the volume of information, an optical disk has recently been required to have a higher recording density. A higher recording density of an optical disk has been achieved by increasing a linear recording density in an information recording layer of the optical disk or decreasing a track pitch. To support for the achievement of a higher recording density of an optical disk, it is necessary to decrease a diameter of a light beam condensed onto the information recording layer of the optical disk.
As a method for decreasing a diameter of a light beam, there are (i) a method in which a light beam having a short wavelength is used and (ii) a method in which an NA (Numerical Aperture) of an objective lens is increased, which objective lens is a condensing optical system of an optical pickup device which records and reproduces an optical disk.
As to the method of using the light beam whose wavelength is short, a technique using a blue-violet semiconductor laser whose wavelength is 405 nm is put into practical use. This allows a red semiconductor laser having a wavelength of 650 nm, which is a light source generally used for DVD, to be replaced by a blue-violet semiconductor laser having a wavelength of 405 nm.
As to the method of increasing the NA of the objective lens, there was conventionally proposed a method in which an objective lens made up of two lenses (a pair of lenses) is used. However, a technique using a single objective lens having a high NA such as 0.85 is put into practical use as a result of an advanced lens designing technique and an advanced lens manufacturing technique.
Generally, the optical disk is arranged such that its information recording layer is covered by a cover glass so as to be protected from dusts and free from any damages. Thus, a light beam passing through the objective lens of the optical pickup device further passes through the cover glass so as to be condensed and focused on the information recording layer below the cover glass.
When the light beam passes through the cover glass, spherical aberration (SA) occurs. The spherical aberration SA is expressed as follows:SA∞(d/λ)·NA4  (1)As expressed above, the spherical aberration SA is proportional to the thickness d of the cover glass and fourth power of the NA of the objective lens, and the spherical aberration SA is inversely proportional to the wavelength λ of a light source. Generally, the objective lens is designed so that the spherical aberration is offset. As a result of this, the spherical aberration of the light beam passing through the objective lens and the cover glass is sufficiently small.
However, when the thickness d of the cover glass deviates from a predetermined value, the light beam condensed onto the information recording layer has a spherical aberration, so that a diameter of the beam increases. This raises the problem that information is improperly read and written.
Further, according to the foregoing expression (1), the amount of spherical aberration error ΔSA increases as a thickness error Δd of the cover glass increases. From the fact, it is obvious that information on the optical disk is improperly read and written. Further, it is obvious that spherical aberration SA increases as a wavelength λ of a light source decreases.
Moreover, a multilayered optical disk in which information recording layers are laminated for higher density of recorded information in a direction of the thickness of the optical disk has been put into commercial production.
Examples of the multilayered optical disk include DVD (Digital Versatile Disc) and BD (Blu-ray Disc) each of which has two information recording layers. In the optical pickup device for recording/reproducing information on/from such a multilayered optical disk, it is necessary to condense the light beam onto each information recording layer of the optical disk in such a manner that a condensed light spot is sufficiently small.
In the optical disk having plural information recording layers, a distance between a surface (cover glass surface) of the optical disk and one information recording layer is different from a distance between the surface and another information recording layer. Thus, the information recording layers are different from each other in terms of the spherical aberration which occurs at the time when the light beam passes through the cover glass of the optical disk. In this case, according to the expression (1), spherical aberration which occurs between adjacent information recording layers varies (error ΔSA) in proportion to a distance t (corresponding to d) between the adjacent information recording layers.
In case of a DVD having two information recording layers, the NA of the objective lens of the optical pickup device is small (about 0.6). Therefore, it is obvious that a slightly larger thickness error Δd in the cover glass has little influence on the of the spherical aberration error ΔSA according to the expression (1).
Thus, in the DVD device using a conventional optical pickup device whose NA is about 0.6, the thickness error Δd in the cover glass of the DVD causes small spherical aberration error ΔSA. This makes it possible to condense the light beam onto each information recording layer in such a manner that a condensed light spot is sufficiently small.
However, even with the same thickness errors Δd in the cover glasses, a large spherical aberration SA occurs with increase of the NA. For example, if the NA is changed from 0.6 to 0.85, the spherical aberration SA becomes 4 times greater. Furthermore, even with the same thickness errors Δd in the cover layers, a large spherical aberration SA occurs with decrease of a wavelength. For example, if the wavelength λ is changed from 650 nm to 405 nm, the spherical aberration SA becomes about 1.6 times greater. Thus, in the BD using a short wavelength light source and a high numerical aperture, the spherical aberration SA is about 6.4 times greater than that of the DVD.
Similarly, in the case of the multilayered optical disk, even with the same distances t between the adjacent information recording layers, the spherical aberration difference (error ΔSA) increases as the NA of the objective lens of the optical pickup device. For example, if the NA is changed from 0.6 to 0.85, the spherical aberration error becomes about 4 times more greatly. Therefore, according to the expression (1), it is obvious that the error in spherical aberration between the information recording layers increases as the NA increases to 0.85, for example.
In this way, the objective lens having a high NA raises such a problem that the spherical aberration error is not ignorable and would drop accuracy in reading information. Thus, it is necessary to correct the spherical aberration in order to realize higher-density recording with the objective lens whose NA is high.
For example, Patent Document 1 and other documents discloses, as a technique for correcting the spherical aberration, a technique in which: a hologram element divides returning light beams, having been reflected by the optical disk and being condensed onto the hologram element, into a first light beam that is near an optical axis of the bundle of light beams and a second light beam that is outer than the first light beam (near the periphery of the bundle of light beams), and a difference between a position at which the first light beam is condensed and a position at which the second light beam is condensed is used to detect and correct the spherical aberration.
With reference to FIG. 14, the following will describe a general configuration of the optical pickup device disclosed in Patent document 1.
In an optical pickup device 200, a hologram element 210, a collimator lens 203, and an objective lens 204 are disposed in an optical axis OZ that is formed between an light beam emission surface of a semiconductor laser 201 and a light beam reflection surface of the optical disk 206. A light detector 207 is disposed at a position where diffracted light from the hologram element 210 is condensed. Note that the hologram element 210 may be replaced by a hologram element 220 having a division pattern (hologram pattern) which is different from that of the hologram element 210.
More specifically, in the optical pickup device 200, light beams emitted from the semiconductor laser 201 pass through the hologram element 210 as zero order diffracted light, and the zero order diffracted light is converted into parallel light by the collimator lens 203, and the parallel light is condensed onto an information recording layer 206c or 206d, which will be described later, on the optical disk 206 via the objective lens 204.
Meanwhile, light beams reflected by the information recording layer 206c or 206d of the optical disk 206 pass through the objective lens 204 and the collimator lens 203 in this order and become incident on the hologram element 210, and the incident light is diffracted by the hologram element 210 so as to be condensed on the light detector 207. The light detector 207 is disposed at a position where positive first-order light from the hologram element 210 is focused.
The optical disk 206 is made up of a cover glass 206a, a substrate 206b, and the above-mentioned two information recording layers 206c and 206d, which are formed between the cover glass 206a and the substrate 206b. That is, the optical disk 206 is an optical disk having two layers. The optical pickup device 200 causes light beams to be condensed onto the information recording layer 206c or 206d, so as to reproduce information from the information recording layer 206c or 206d and record information onto the information recording layer 206c or 206d. 
A division pattern of the hologram element 210 used in the first conventional example will be described in detail with reference to FIG. 15. The hologram element 210 has the following three regions: a first region 210a; a second region 210b; and a third region 210c. 
The first region 210a is a region which is surrounded by a line D11 extending in a radial direction orthogonal to the optical axis OZ and an arc of a first semicircle E11 (whose radius is r11) centered about the optical axis OZ. Further, the second region 210b is surrounded by an arc of a second semicircle E12 (whose radius is r12; r12>r11) centered about the optical axis OZ, the arc of the first semicircle E11 (whose radius is r11), and the line D11. The third region 210c is a region which is surrounded by an arc of a third semicircle E13 (whose radius is r12) and the line D11. The third semicircle E13 is located opposite to the first semicircle E11 and the second semicircle E12 (located in a negative Y direction in FIG. 15) with respect to the line D11. It is possible to maximize detection sensitivity of a spherical aberration error signal (hereinafter referred to as “SAES”) when r11 is set to r11=0.7r10 where r10 (r12>r10>r11) is a radius of an effective radius of the light beam 208 determined by an aperture of the objective lens 204 (FIG. 14) on the hologram element 210.
Next, a division pattern of the hologram element 220 used in the second conventional example will be described in detail with reference to FIG. 16. The hologram element 220 has the following three regions: a first region 220a; a second region 220b; and a third region 220c. The first region 220a is a region surrounded by a line D21 extending in a radial direction orthogonal to the optical axis OZ, a line D22 at a distance h5 away from the line D21 in a Y direction, and arcs E21 and E22 of a circle (whose radius is r12) centered about the optical axis OZ. The second region 220b is a region surrounded by the line D22 and an arc E23 of the circle (whose radius is r12) centered about the optical axis OZ. The third region 220c is a region surrounded by the line D21 and an arc E24 of a semicircle (whose radius is r12) centered about the optical axis OZ.
Assume that r10 (r12>r10) is an effective radius of the light beam 208 determined by an aperture of the objective lens 204 (FIG. 14) on the hologram element 220. In this case, a distance h5 between the lines D21 and D22 is set to h5=0.6r10. Thus, since the first region 220a and the second region 220b are divided by the line D22 extending in the radial direction, no influence of an objective lens shifting at the time of tracking control occurs. This causes little variation in detection sensitivity of the SAES.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 157771/2002 (Tokukai 2002-157771; published on May 31, 2002)